mongo & duplicate _id's

Adam$ script/console 
Loading development environment (Rails 2.3.4)
>> Mongo::VERSION
=> "0.15.1"
>> #MongoMapper::VERSION == 0.5.5
?> document = Class.new do
?>   include MongoMapper::Document
>>   set_collection_name 'duplicate_id_test'
>> 
?>   key :data, String
>> end
=> #<Class:0x10340c9b0>
>> document.collection.clear
=> #<TCPSocket:0x1033c3670>
>> 
?> doc = document.new :data => 'howdy do!'
=> #<#<Class:0x10340c9b0> data: "howdy do!", _id: nil>
>> doc.save
=> true
>> doc.inspect
=> "#<#<Class:0x10340c9b0> data: "howdy do!", _id: 4adfddf80a0aec3028000001>"


#FROM mongo console
> db.duplicate_id_test.count()
1
> db.duplicate_id_test.findOne()
{
	"_id" : ObjectId("4adfddf80a0aec3028000001"),
	"_id" : ObjectId("4adfddf80a0aec3028000001"),
	"data" : "howdy do!"
}